The invention relates generally to magnetic disk drive storage systems and, more specifically, to a method and an apparatus for using separate servo and data read elements to allow for more efficient positioning of a transducer.
A magnetic disk drive system is a digital data storage device that stores digital information within concentric tracks on a storage disk (or platter). The storage disk is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a substantially constant rate. To write data to or read data from the disk, a magnetic transducer is positioned above a desired track of the disk while the disk is spinning.
Writing is performed by delivering a write signal having a variable current to a transducer while the transducer is held close to the rotating disk over the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored.
Reading is performed by sensing magnetic polarity transitions previously written on tracks of the rotating disk with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the magnetic signal into an analog read signal which is amplified in a preamplifier, whereafter the signal is delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that, after additional processing, can be recognized by a host computer system external to the drive.
The transducer contains a read element and a write element to respectively perform the functions of writing to and reading from the disk. Some transducers contain dual-purpose elements which can both write to and read from the disk, but modern transducers separate the read element from the write element for reasons explained below.
Portions of a standard disk drive, generally designated 100, are illustrated in FIGS. 1A and 1B, where FIG. 1A is a top view of the disk drive 100 and FIG. 1B is a sectional side view thereof The disk drive comprises disks 104 that are rotated by a spin motor (not shown). The spin motor is mounted to a base plate (not shown). Data is stored on magnetic material which coats the two surfaces 108 of the disk 104. An actuator arm assembly 112 is also mounted to the base plate.
The actuator arm assembly 112 includes a transducer 116 mounted to an actuator arm 124. The actuator arm 124 rotates about a bearing assembly 128. The actuator arm assembly 112 cooperates with a voice-coil motor (VCM) 132 which moves the transducer 116 relative to the disk 104. The spin motor and voice-coil motor 132 are coupled to a number of electronic circuits mounted to a printed circuit board (not shown) which control their operation. A number of wires 136, among other things, are used to couple the transducer 116 to the read channel (not shown in FIG. 1A). These wires are routed from circuitry within the drive, across the actuator arm assembly 112 and to the transducer 116. An analog read signal and an analog write signal are transported by these wires 136. The analog read signal is amplified by a preamplifier 140 before it is further processed by other circuitry (not shown) into a digital representation of the data stored on the disk 104. The preamplifier 140 is typically located on the actuator arm assembly 112 and positioned as close to the transducer 116 as practical so that noise may be reduced. After the preamplifier 140, the amplified analog read signal is passed to other circuitry which may include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device, among other things.
As shown in FIG. 1B, each of the plurality of disks 104 has two sides, with magnetic material 108 on each of those sides. Two actuator arm assemblies 112 are provided for each disk 104. To position the transducer 116, the VCM 132 moves all actuator arms 124 in unison relative to their respective disks 104. The VCM 132 makes position adjustments to the pivotally connected actuator arms 124 so that a particular transducer is centered over a data track 144 (see FIG. 1A). As is well understood in the art, movement of each actuator arm 124 can be independently optimized for imperfections in the arcuate geometry of each data track 144 on the actuator arm""s corresponding magnetic surface 108.
Referring to FIG. 1A, data is stored on the disk 104 within a number of concentric data tracks 144 (or cylinders). Each data track 144 is divided into a plurality of sectors, and each sector is further divided into a data region 148 and a servo region (or servo sector) 152.
Servo sectors 152 are used to, among other things, provide transducer position information so that the transducer 116 can be accurately positioned by the actuator arm 124 over the data track 144, such that user data can be properly written onto and read from the disk 104. The data regions 148 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten. Because servo sectors are embedded into each data track 144 on each disk 104 between adjacent data regions 148, the type of servo-scheme shown in FIG. 1A is known by those skilled in the art as an embedded servo scheme (or sectored servo scheme).
A more detailed view of a transducer, generally designated 116, used for reading and writing magnetic polarity transitions to a magnetic media (not shown) is illustrated in FIG. 2. Referring to the figure, portions of the transducer 116 which face the magnetic media are shown. The part of the transducer 116 shown in this view is commonly called the air bearing surface. The transducer 116 includes a write element 200, write gap 204, first shield 208, second shield 212, read gap 216, and magnetoresistive (MR) read element 220. Unlike some early inductive transducers, the depicted transducer 116 has separate read and write elements. Magnetoresistive (MR) strips are commonly used in read elements because they change resistance when exposed to a magnetic field, and this change in resistance is relatively easy to sense. It should be noted that the read element 220 is used for reading both servo and data regions. It is further noted that the write element 200 typically has a width 224 which is greater than a width 228 of the MR read element 220. For example, the width 224 of the write element 200 might be twice the width 228 of the read element 220. The reason for this width variance is explained below.
As part of the writing process, a variable current is used to induce magnetic flux across the write gap 204 between the write element 200 and the first shield 208. The write element 200 and first shield 208 act as poles for an electromagnet which induces magnetic flux across the write gap 204. The direction of the variable current defines the direction in which the magnetic flux will be oriented across the write gap 204. In some simple recording systems, flux polarized in one direction across the write gap 204 will record a binary xe2x80x9conexe2x80x9d on the magnetic media while flux polarized in the opposite direction will record a binary xe2x80x9czero.xe2x80x9d In most recording systems, a change in the direction that the flux travels across the gap 204 is interpreted as a xe2x80x9conexe2x80x9d while the lack of a change is interpreted as a xe2x80x9czero.xe2x80x9d As the magnetic material on the disk surface 108 (shown in FIG. 1A) travels under the transducer 116 in the direction shown by arrow 232, a series of digital xe2x80x9conesxe2x80x9d and xe2x80x9czerosxe2x80x9d can be written within the data track 144 (shown in FIG. 1A).
When reading, the magnetic polarity transitions, previously written onto the magnetic media, are coupled to the transducer 116 in order to recover the stored digital data. When a magnetic polarity transition in the magnetic media passes under the transducer 116, the read element 220 will each generate a signal in response to the changing magnetic field which corresponds to a previously recorded data bit. This signal is called an analog read signal. A preamplifier 140 (shown in FIG. 1A) is used to provide low noise amplification of the analog read signal. Conversion of the analog read signal back into a digital signal is performed within a read channel, after which it is passed to an exterior environment such as a computer. During the read process, the first and second shields 208, 212 form a read gap 216 which serves to focus the flux for a particular magnetic polarity transition onto the read element 220 by shielding the read element 220 from other sources of magnetic flux. In other words, extraneous magnetic flux is filtered away from the read element 220 by the shields 208, 212.
FIG. 3 shows a portions of a number of data tracks 144 drawn in a straight, rather than arcuate, fashion for ease of depiction. As is well-known, data tracks 144 on magnetic disks 104 (as depicted in FIG. 1A) are circular. Referring again to FIG. 3, each data track 144 has a centerline 300. To accurately write data to and read data from the data region 148 of the data track 144 while the disk travels in a direction denoted by the arrow 232, it is desirable to maintain the transducer 116 (see FIG. 1A) in a relatively fixed position with respect to a given track""s centerline 300 during each of the writing and reading procedures.
With reference to FIGS. 1-3, to assist in controlling the position of the transducer 116 relative to the track centerline 300, the servo region 152 contains, among other things, servo information in the form of servo patterns comprised of two or more groups of servo bursts, as is well-known in the art. First and second servo bursts 304, 306 (commonly referred to as A and B servo bursts, respectively) are shown in FIG. 3 for each data track 144. Servo bursts 304, 306 are accurately positioned relative to the centerline 300 of each data track 144, and are typically written on the disk 104 during the manufacturing process using a servo track writer (xe2x80x9cSTWxe2x80x9d). The STW alternatively writes a number of A bursts 304 in a concentric circle and then a number of B bursts 306 in a concentric circle until all data tracks 144 have servo information embedded therein. The concentric circle of either A or B bursts 304, 306 is defined herein as a servo track. Unlike information in the data region 148, servo bursts 304, 306 may not normally be overwritten or erased during operation of the disk drive 100.
As the transducer 116 is positioned over the data track 144, it reads the servo information contained in the servo regions 152 of the track, one servo region at a time. The servo information is used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer 116 and the track centerline 300. The PES signal is provided as an input to a servo control loop which performs calculations and outputs a servo compensation signal which controls the VCM 132 to place the transducer 116 over a particular position relative to the track centerline 300. When a write function is desired, the dual-purpose transducer 116 reads servo information from the servo region 152, is positioned over the track centerline 300 in the manner described above, and then writes to the disk 104 when the transducer 116 is over the data region 148.
As mentioned above, the read element 220, shown in FIG. 2, reads information from both the servo region 152 and data region 148. The dual-purpose nature of the read element 220 requires transducer designers to compromise between optimizing the read element 220 for reading servo information or for reading user data, as explained more fully below.
Referring to FIG. 4, the transducer 116 and a portion of the analog read channel is illustrated in block diagram form. The transducer 116 contains a write element 200 and a dual-purpose servo/data read element 220. An analog write signal 400 is provided to the write element 200, and an analog read signal 404 is received from the read element 220. To provide low noise factor amplification, the analog read signal 404 is provided to a preamplifier 140. An amplified analog read signal 408 is sent from the preamplifier 140 to a demultiplexor 412 where a servo read signal 416 and a data read signal 420 are produced under the control of a select line 424. Typically, the preamplifier 140 is located on the actuator arm assembly 112 (see FIGS. 1A and 1B) while the demultiplexor 140 is located in the read channel. The select line 424 is provided by other circuitry within the read channel and controls the demultiplexing of the amplified read signal 408. In this way, the dual-purpose servo/data read element 220 provides both the servo read signal 416 and the data read signal 420.
Referring once again to FIGS. 1-3, the geometric relationship of data tracks 144 and transducer 116 is explained. The data region 148 has a width 312 (see FIG. 3) in the radial direction of the disk 104 generally equal to the width 224 (see FIG. 2) of the write element 200. As an artifact of the write process, erase bands 316 (see FIG. 3) are created between each data track 144. The erase bands 316 are considered wasted space since they cannot store user data. To reliably position the read element 220 over the centerline 300 and avoid the erase bands 316, one skilled in the art can appreciate the desirability of having an read element width 228 (see FIG. 2) smaller than the data region width 312. In other words, the smaller the read element width 228 with respect to the data region width 312 (see FIG. 3), the more the likely the read element 220 will be positioned over a portion of the data region 148 with a magnetic signal of sufficient amplitude to reliably read the user data.
As shown in FIG. 1A, circular data tracks 144 extend from an inner usable radius of the disk (xe2x80x9cinner diameterxe2x80x9d or xe2x80x9cIDxe2x80x9d) 156 to an outer usable radius of the disk (xe2x80x9couter diameterxe2x80x9d or xe2x80x9cODxe2x80x9d) 160. The servo regions 152 for each data track 144 are generally aligned radially, and the servo regions are generally the same size regardless of the radial positioning of the servo region 152. A servo burst width 320 (see FIG. 3) is approximately equal to the data region width 312 plus an erase band width 324. As can be appreciated, toward the distal dimensions of the disk the data regions 148 are physically longer because the servo regions 152 remain the same size as the circumference of the data track 144 grows larger.
Along a given radius of the disk there is a ratio between the total number of A and B servo bursts 304, 306 and the data tracks 144. In FIG. 3, each data track 144 requires half of the A burst and half the B burst to accurately position the transducer 116 over the track centerline 300. Accordingly, the ratio of A and B servo burst 304, 306 to data tracks 144 for this configuration is one to one.
With reference to FIG. 5, a block diagram of the read electronics for a conventional two disk (i.e., four surface) drive system is shown. Each surface 108 (see FIG. 1B) of the two disks 104 has a corresponding transducer 504, 508, 512, 516 which reads both servo and user data from that surface 108. A read element 220 (see FIG. 2) in a first transducer 504 produces a first analog read signal 520 which is amplified in a first preamplifier 536, whereupon a first amplified analog read signal 552 is produced. Similarly, a second, third and fourth preamplifiers 540, 544, 548 respectively amplify each of a second, third and fourth analog read signals 524, 528, 532 to produce corresponding second, third and fourth amplified analog read signals 556, 560, 564. In this way, the amplified read signal 552, 556, 560, 564 is produced for each disk surface 108 (see FIG. 1A).
In conventional disk drives 100 with embedded servo sectors, only one disk surface 108 is read from at a time. This means only one of the first, second, third, and fourth amplified analog read signals 552, 556, 560, 564 requires decoding to read from the single disk surface 108. This allows multiplexing 572 of the first, second, third, and fourth amplified analog read signals 552, 556, 560, 564 into a single read channel 580. Based upon the select inputs 568, the multiplexer 572 routes one of the first, second, third, and fourth amplified analog read signals 552, 556, 560, 564 to a selected analog read signal path 576. The read channel 580 takes the signal coupled to the selected analog read signal path 576 and processes the signal to produce either a digital representation of the user data or the servo information. In this way, each of the four disk surfaces 108 is read by a single read channel 580.
As those skilled in the art can appreciate, when the read element width 228 is less than the servo burst width 320, conventional position error techniques will produce PES signals with a non-linear response. The non-linear response makes it difficult to determine the centerline 300 of the data track 144. This problem in the prior art is best illustrated with examples.
FIGS. 6A-C show three different radial positions for the read element 220 with respect to servo region 152 and also show the respective A and B burst signals produced by the read element 220 at each radial position. In the depicted examples, the read element width 228 is less than the servo burst width 320. To determine off-track position, the A burst signal produced over the A servo burst pattern 304 is compared to the B burst signal produced over the B servo burst pattern 306 as the disk 104 rotates in the direction of the arrow 232. The first example in FIG. 6A depicts the read element 220 straddling the division between A and B servo bursts 304, 306 which produces the A and B burst signals shown. In the second example shown in FIG. 6B, the read element 220 is at a radial position toward the top edge of the A servo burst 304 which produces the A and B burst signals shown. The resulting A and B burst signals from the second example in FIG. 6B should be contrasted with the A and B burst signals produced in the third example shown in FIG. 6C. Although the read element 220 is at a radial position closer to the centerline 300 of the data track 144 in FIG. 6C, the A and B burst signals produced are indistinguishable from that of FIG. 6B.
As can be appreciated, the non-linearity in the A and B burst signals at different radial offsets demonstrated in FIGS. 6A-C makes off-track position determination difficult. This problem is solved by having the read element width 228 be greater than or equal to the servo burst width 320. However, enlarging the read element width 228 makes the positioning of the read element 220 while avoiding the erase bands 316 (see FIG. 3) more difficult, as explained above.
With reference to FIG. 7, a quadrature servo burst pattern is depicted with an arrow 232 defining the direction the disk rotates. To solve the non-linear response problems when the read element width 228 (see FIG. 2) is smaller than the servo burst width 320 additional servo bursts patterns are added by staggering four groups of servo bursts 304, 306, 704, 708 in a manner commonly called quadrature servo burst patterns 712. Additional third and fourth servo bursts 704, 708 (commonly called C and D servo bursts, respectively) are added to the A and B servo bursts 304, 306. In this way, the read element width 228 can be reduced to half the servo burst width 320 and still avoid nonlinear responses in the off-track detection. There is a ratio between the total number of A, B, C, and D servo bursts 304, 306, 704, 708 along a given radius and data tracks 144 of two to one for quadrature servo burst configurations. However, while adding more servo bursts 304, 306, 704, 708 per data track 144 may allow for linear off-track position determinations and more accurate read operations, the writing of the quadrature servo patterns 712 increases the drive production time and equipment costs needed for additional servo track writers while decreasing manufacturing throughput. Additionally, consuming space on the surface 108 of the magnetic disk 104 with additional servo bursts leaves less space available for user data.
Other conventional systems have attempted to solve the nonlinear off-track position problem by using the write element to read servo bursts which allows dedication of the read element for reading user data. In other words, the write element serves a dual role as a data writer and a servo reader. Since the read element is dedicated to the reading of user data, the width of the read element can be made optimally small. Unfortunately, conventional write elements do not efficiently perform read functions which makes this solution impractical.
As noted in the above discussion, the width of the write element generally defines the width of the data track 144, and the width 320 (see FIG. 3) of the servo bursts in systems, which only use the A and B bursts 304, 306, is generally equal to the width of the data track 144 plus the width 324 of the erase band 316. As can be appreciated, the dual purpose data writer and servo reader will experience a non-linear response to the servo bursts since the width 224 of the write element is necessarily smaller than the width 320 of the servo burst.
With reference to FIG. 8, the use of a center-tapped reader 802 has been suggested to bifurcate a single MR strip 818 into a user data read element and a servo read element. A first conductor 806 supplies a first current 822 to the MR strip 818 and a third conductor 814 supplies a second current 826 to the MR strip 818, whereupon a second conductor 810 returns a third current 834. The third current 834 returns a sum of the first and second currents 822, 826 and is common to the bias path of the data read element and the servo read element. It should be noted that the bias currents in the data read element and servo read element necessarily flow in opposite directions.
Center-tapped read elements suffer from poor servo read sensitivity since the placement of the second conductor 810 on the MR read element 818 creates a dead spot. As can be appreciated by those skilled in the art, a metallic conductor 810 in a central portion of the MR read element 818 serves to make the read element insensitive to the magnetic signal stored on the data track 144. In other words, the affected portion of the MR element 818 is not sensitive to magnetic fields incident upon the dead spot.
Center-tapped MR read elements are also undesirable because proper biasing of the bifurcated reader 802 is difficult. As noted above, the first and second currents 822, 826 flow in opposite directions because of the common center conductor 810. Those skilled in the art can appreciate that magnetic biasing is aligned with the current flow such that magnetic biasing of each half of the MR strip 818 should also be in opposite directions. However, it is difficult to bias permanent magnets, which provide the magnetic biasing, in opposite directions during manufacture. Accordingly, there is a need to provide separate data and servo read elements which do not experience dead spots in a configuration which allows for proper magnetic biasing.
In summary, it would be desirable to develop a transducer positioning system which: (1) optimizes the transducer for reading user data; (2) also optimizes the transducer for reading servo information while minimizing the number of servo bursts; and, (3) avoids the deficiencies of center-tapped MR read elements, including their dead spots and biasing problems.
It is an object of the present invention to develop a transducer positioning system which: (1) optimizes the transducer for reading user data; (2) also optimizes the transducer for reading servo information while minimizing the number of servo bursts; and, (3) avoids the deficiencies of center-tapped MR read elements, including their dead spots and biasing problems.
A magnetic storage system is disclosed which has one or more rotating disks. In one embodiment, the system includes a first transducer and a second transducer operatively associated with a respective first surface or second surface. Each of the first and second surfaces has a data track with a data region and an embedded servo sector. The first transducer has a data reader which includes a first MR strip and a servo reader which includes a second MR strip. The first MR strip is electrically isolated from the second MR strip. The first transducer performs a servo reading operation of the servo sector while the second transducer performs either: (1) a data writing operation to the data region, or (2) a data reading operation from the data region.
Also disclosed is a method for reading within a magnetic storage system. The storage system includes a first magnetic surface, a data reader and a servo reader. The first magnetic surface includes a first data region and a first servo sector. The data reader is not coupled to the servo reader. Servo information is read with the servo reader so that an off-track position of the data reader can be determined. After the off-track position is known, the data reader is moved to compensate for the off-track position. Once the data reader is properly positioned, the data reader retrieves user data from the first data region of the first magnetic surface.